Technical Field
The present disclosure relates to an active noise cancelling device and to a method of actively cancelling acoustic noise.
Description of the Related Art
As is known, active noise cancelling is becoming more and more used to improve performance of audio systems, such as headphones, headsets, hearing aids, microphones and the like. This trend is also encouraged by recent developments in the field of microelectromechanical systems (MEMS), which provided extremely effective and sensitive devices, such as microphones and speakers, having the additional advantage of very low power consumption.
Active noise cancelling essentially consists of detecting acoustic noise produced by noise sources through a microphone at a given location, and using a feedback control based on microphone response to produce acoustic waves that tend to cancel noise by destructive interference in a band of interest (e.g., an audible band roughly comprised between 16 Hz and 16 kHz).
Most of known active noise cancelling systems are based on analog circuitry, namely analog filters, because it is normally possible to achieve lower phase delay compared to digital solutions. Filters are in fact included in the feedback control loop and phase delay is well-known to be a critical aspect for stability of feedback system.
Apart from a general trend toward digital solutions, analog active noise cancelling systems present some limitations in terms of poor flexibility, accuracy requirements of components, power consumption, area occupation and, in the end, cost. For example, it is quite difficult, or even impossible at all, sometimes, to provide for adjustable filter response and every component, including resistors, should be accurately trimmed to ensure expected performance. Thus, purely analog implementations are not ideally suited to improve miniaturization and flexibility of use.
On the other hand, known solutions that involve digital processing based on conventional chains of IIR filters may suffer from low sampling rate typical of audio systems (e.g., 48 kHz) and phase delay, which in turn may undermine stability, as already mentioned. Other active noise cancelling systems envisage higher sampling rates, but these solutions are normally demanding in terms of processing capability. Devices that meet processing requirements (e.g., Digital Signal Processors, DSP) are usually costly and power consuming.